1. Field of the Invention
The present invention relates to an image sensor comprising an array of photosensitive cells, or pixels, arranged in rows and/or in columns and obtained in a CMOS-type technology. Such an image sensor or photodetector is intended to be used in image shooting devices such as, for example, cameras, cell phones, or digital photographic cameras.
2. Discussion of the Related Art
FIG. 1 is a cross-section view of a pixel (Pix) of an image sensor formed in and on a substrate 7, for example, silicon. The pixel is associated with a portion of surface 8 of substrate 7 which, in top view, generally has the shape of a square or of a rectangle. The pixel comprises a photosensitive active area PH generally corresponding to a photodiode capable of storing a quantity of electric charges according to the received light intensity. Photosensitive area PH is generally located at the center of the portion of surface 8 associated with the pixel. Substrate 7 is covered with a stack of transparent insulating layers 9, 11, 12, 13 which may be, as an example, alternately silicon oxide and silicon nitride. Conductive tracks 14, formed between insulating layers 9 and 11, and conductive vias 16, formed in insulating layer 9, especially enable addressing photosensitive area PH and collecting electric signals provided by photosensitive area PH. Conductive tracks 14 and conductive vias 16 are generally metallic. As an example, aluminum, tungsten, copper, and metal alloys may be mentioned. Such materials are opaque and possibly reflective. Conductive tracks and vias may be provided at the level of insulating layers 11, 12, 13. In a color sensor, a color filter element 17, for example, an organic filter, is arranged on the stack of insulating layers 9, 11, 12, 13 in line with the pixel. Color filter elements 17 are generally covered with a planarized equalization layer 18 which defines an exposure surface 19 exposed to light.
Photosensitive area PH generally does not extend over the entire portion of surface 8 of substrate 7 associated with a pixel. Indeed, a portion of this surface is dedicated to devices for addressing photosensitive area PH and reading from it, not shown in FIG. 1, and which for example comprise MOS transistors. To redirect the maximum amount of light which reaches the portion of exposure surface 19 associated with the pixel towards photosensitive area PH, a microlens 21, of optical axis Δ, is arranged on equalization layer 18, opposite to photosensitive area PH to focus the light rays towards photosensitive area PH. As an example, the travels of two light rays R1, R2 are schematically shown in FIG. 1. Conductive track 14 and conductive vias 16 are arranged to avoid hindering the passing of the light rays. Microlens 21 is for example obtained by covering equalization layer 18 with a resin. The resin is etched to delimit distinct resin blocks. The resin blocks are then heated. Each resin block then tends to deform by flowing to obtain a convex external surface 22. The flow step is followed by a microlens reticulation step. To direct the maximum amount of light towards photosensitive area PH, microlens 21 generally has a square or rectangular base which practically takes up the entire portion of exposure surface 19 associated with the pixel.
For applications such as mobile telephony, the present tendency is to decrease the dimensions of pixels to be able to integrate a greater number thereof on a same surface of a substrate. To limit the decrease in the dimensions of photosensitive area PH, it is desired to pool at least part of the addressing and reading devices, for example, MOS transistors, between adjacent pixels. To pool MOS transistors between adjacent pixels, it may be necessary not to form photosensitive areas PH at the center of pixels. Thereby, the center of photosensitive area PH is offset with respect to optical axis Δ of microlens 21.
There then is a risk that only part of the light focused by microlens 21 will be received by photosensitive area PH. The image sensor sensitivity may then decrease. Further, since the position of photosensitive area PH with respect to the center of a pixel can vary from one pixel to the other, two pixels illuminated in the same way may provide different signals, given that their photosensitive area does not receive the same amount of light. Further, since the position of photosensitive area PH with respect to the center of a pixel may vary from one pixel to the other, it is not possible to offset each microlens 21 to align it back on the corresponding photosensitive area PH, short of forming partially overlapping lenses or of decreasing the dimensions of microlenses 21. However, decreasing the dimensions of microlenses 21 means that part of the light reaching exposure surface 19 will not be directed towards photosensitive areas PH, thus resulting is a sensitivity loss of the sensor.